Shading correction data fetching in the conventional type of image forming apparatus has been executed, for instance, in the way as described below. Namely, after a print switch is pressed down, white data for shading correcting is fetched and then an operation for forming an image is executed.
Also to reduce effects caused by change in a quantity of light from a halogen lamp for illuminating a draft during lapse of time when copying is executed continuously, white data is fetched each time one image is formed.
The reference technological documents relating to the present invention include, for instance, Japanese Patent Laid-Open Publication No.50822/1987 disclosing an "Image Forming Apparatus" in which a light quantity detection sensor is used to detect change during a build-up time of a light source and scanning for exposure is started after a stable output from the light source has been obtained.
Also Japanese Patent Laid-Open Publication No.183259/1987 discloses an image forming apparatus in which an operating speed of a driving unit is set to a lower speed and a time required for a draft image to be fed to a read position is extended when a quantity of light detected by a light quantity measuring means is small.
Furthermore, in a digital reading system Japanese Patent Laid-Open Publication No.177765/1990 discloses an "Image Reading Apparatus" in which light from a fluorescent lamp is led to a photo diode through an optical fiber, and the output is computed and used as a reference input for an A/D converter.
However, in the conventional type of image forming apparatuses as described above, there have been the problems as described below. Detailed description is made hereinafter.
At first, FIG. 17 and FIG. 18 show image data output values for R, G and B lights in a state where the system is cool due to change in a quantity of light from a halogen lamp and in a state where the system has been warmed up due to change in a quantity of light from a halogen lamp respectively. Data is fetched into a shading white memory in 0.69 sec after the halogen lamp is lit, and comparison between the cool state shown in FIG. 17 and the warmed-up state shown in FIG. 18 shows that the image data difference is higher in the cool state by 3.2% for R, 3.3% for G, and 4.8% for B.
This phenomenon occurs because a section not emitting light (Mo ring) in the Halogen lamp is heated when the halogen lamp is lit and the resistance value increased. For instance, when 6 pieces of Mo ring each having a diameter of 0.43 mm and a length of 44 mm are used, the resistance is 0.017 .OMEGA..times.6 pieces .noteq.0.1 .OMEGA..
On the other hand, once a lamp is lit and temperature of an Mo ring goes up to 200.degree. C., the resistance becomes 0.2 .OMEGA.. Assuming that the rated current is 2.6 A, a voltage drop due to the Mo ring is 0.26 V, and a quantity of light then is
{(75-0.26)/75}.sup.3.38 =0.988
meaning that the quantity of light has changed by 1.2%, it should be noted that a change in actually measured values shown in FIG. 17 and FIG. 18 is larger.
Thus change in a quantity of light from a halogen lamp give effects to an operation for fetching shading white data first in a copying sequence in an actual machine (in a state where the machine is still cool) as well as to operations for fetching shading white data second time and on (in a state where at least one scan has been executed and the halogen lamp has been warmed up).
FIG. 19 and FIG. 20 are timing charts each showing operations for fetching shading correction data after a print switch is pressed down as well as for scanning in the conventional type of image forming apparatus.
In FIG. 19, after power is turned ON, the image forming apparatus is initialized, reload and fixing functions are initialized, and then a scanner initial setting command is transmitted from the main control section, when draft image read is enabled because the analog system in the image forming apparatus is automatically set up.
When the system enters a stand-by state until the print switch is pressed down, and during this period, the halogen lamp is kept in a cool state. Then, if the print switch is pressed down and a shading start signal is transmitted from the main control section, and operation for fetching shading correction data is executed.
When white data fetching is executed (white memory reset in 8) associated with the shading correction data fetching operation is executed, a quantity of light from the halogen lamp shifts to a slightly higher level and values of read white data becomes higher. If a scanner start signal is transmitted from the main control section in this state, a scanning operation is started, but as the halogen lamp has been warmed up because of the shading correction data fetching operation executed immediately before, a quantity of light becomes lower as compared to that in the cool state.
The shading correction data fetching operation is executed once each time an image is formed, as shown in FIG. 20, and the halogen lamp is warmed up after the first operation is executed, so that values of fetched white data in the operation executed second time and on are different from that executed first time. For this reason values of image data after shading correction fetched in first to fourth scans for a sheet of copy in a repeated copying operation becomes lower as compared to those fetched during scans for a second sheet of copy, and this difference appears as change in image density.
Thus, in the conventional technology, change is generated in a quantity of light from a halogen lamp as a light source due to the internal temperature characteristics, and especially the difference between a high-lighted section and particularly a background section of a first sheet of copied image and those of a second sheet of copied image is substantially large, which disadvantageously makes it impossible to obtain a high quality image.
Also in the conventional technology, a long time, is required until a quantity of light emitted from a light source is stabilized, which in turn makes a lead time for copying operation or a time required for first copy disadvantageously longer.
Also in the image reading apparatus disclosed in Japanese Patent Laid-Open Publication No.50822/1987, amplitude of light emission from the image illuminating light source is detected by a light sensor, and an operation for reading an image is started after the amplitude of light emission is stabilized, but with this configuration, a dedicated light sensor is required, so that a number of necessary parts increases, the productivity becomes lower, reduction of size and weight of the apparatus becomes difficult, and in addition a long time is required until an operation for reading an image is required, and for the reasons as described above, the configuration is not preferable.
Also in the image reading apparatus disclosed in Japanese Patent Laid-Open Publication No.183259/1987, a quantity of light used for reading an image is kept at a constant level by adjusting a scanning speed of an image reading means in response to change in amplitude of light emitted from the image illuminating light source, but actually it is difficult to keep a quantity of light for reading an image at a constant level through adjustment of a scanning speed of an image reading means as described in this document, and also an actual speed when reading an image becomes lower, so that the configuration is not preferable.
In the image reading apparatus disclosed in Japanese Patent Laid-Open Publication No.177765/1990, amplitude of light emission of the image illuminating light source is detected by a light sensor, and an output from this light sensor is used as a reference input for an A/D converter , but with this configuration a dedicated light sensor is required, so that a number of required parts increases, the productivity becomes lower, and reduction of size and weight of the apparatus becomes difficult. Especially, in the image reading apparatus disclosed as an embodiment in this publication, light emitted from a fluorescent lamp is delivered through optical fiber to a photodiode, and for this reason it is considered that the sensitivity in detection may be rather low.